1. Field of the Invention
The invention relates in general to microphones and in particular to a new and useful arrangement for suppressing structure-borne sounds in microphones which are caused by mechanical impacts or vibrations and may lead to corresponding noise signals at the microphone output. Substantially, the invention is concerned with microphones comprising a diaphragm receiving the intelligence sound and fixed to a part which does not directly serve the purpose of converting the signal, and a transducer system corresponding to the diaphragm and necessary for converting the signal.
2. Description of the Prior Art
To abate structure-borne noise caused by mechanical vibrations of a microphone due, for example, to a touch or collision with another object, or to movements of the microphone cable or to the friction of the microphone housing on the user's clothing (Lavalier microphones), substantially two possibilities are given which may be applied individually or in combination;
One possibility is to mount the microphone capsule in the microphone housing resiliently, which arrangement is used frequently. Thorough tests have shown, however, that no satisfactory solution can thereby be obtained. That is, a satisfactory damping is attained only above the natural resonance frequency of the mounting means of the resiliently mounted microphone capsule. Therefore, it would be desirable for an effective damping of these disturbances occurring mostly in the low frequencies, to reduce the natural resonance frequency of the mounting or suspension means as far as possible, preferably below the lowermost frequency to be transmitted.
This, however, is considerably difficult with the modern high-quality microphones having a response range extending from about 30 Hz to 20 kHz, since the lower a mounting resonance frequency is provided, the lesser becomes the mechanical stability of the arrangement, so that tumbling motions and excursions in excess of the capsule in the microphone housing can hardly be prevented, which may lead to mechanical damages of the arrangement. In arrangements of the prior art, the mounting resonance ranges approximately from 50 Hz to 300 Hz, which is still within the usual frequency band to be transmitted. It is therefore obvious that a satisfactory solution of the problem, namely of preventing over a large band the occurrence of disturbing voltages caused by vibrations of the microphone, cannot be obtained with the simple resilient suspension of a microphone capsule from a housing.
The other possibility of abating such disturbing voltages is the compensation method. In the application of this method, a second transmitter system is coupled to the microphone capsule mounted in a housing, which system is inactive relative to the intelligence sound and of such design and arrangement that at the occurrence of mechanical vibrations or shocks, it delivers electric signals which are identical with those delivered by the system provided for converting the intelligence sound. The two outputs are connected to each other in electrical opposition, so that the signals produced by the mechanical vibrations compensate each other.
The transmitter needed for producing the compensating current may be designed, for example, quite symmetrically to the intelligence sound receiver and combined therewith with a screening against the intelligence sound, in a common arrangement which may, in addition, be resiliently mounted in a microphone housing. It is also possible, however, to use for the compensation a transmitter system having one part, namely the part necessary for converting the structure-borne sound into electric signals, rigidly secured to the electroacoustic transducer receiving the intelligence sound and resiliently mounted in the microphone housing, and having its other part rigidly connected to the microphone housing. The electric signal produced by the movements relative to each other of the two parts of the compensating transducer system is delivered in phase opposition to the transducer converting the intelligence sound.
The disadvantage of the prior art compensation arrangement is that they require a second transducer, and perhaps a careful tuning of the two systems to each other over a larger frequency range. In general, the compensation method makes it possible to obtain a satisfactory damping of the disturbing noise, but it must not be overlooked that as compared to the conventional, non-compensated microphones, the transmission factor is reduced to about one half. This drawback may be partly remedied only by improving the efficiency of the sound conversion, which, however, is possible only within certain limits and requires additional expenses.